Potential [E?(ArOH?/0)] give these molecules a strong preference to react by concerted transfer of e- and H+ (HAT). Njus and Kelley used such reasoning to conclude that Vitamin E donates H?as opposed to e- in biological reactions.135 A characteristic of these and other systems that prefer to transfer H?rather than react by stepwise paths (cf., TEMPOH above) is the very large shift of the pKa upon redox change and (equivalently) the large shift of E?upon protonation: for -tocopherol, the pKa changes by 25 units and E?changes by 1.5 V. 5.2.5 Quinones, Hydroquinones and Catechols–The PCET chemistry of hydroquinones and catechols (1,4- and 1,2-dihydroxybenzenes, respectively) is somewhat similar to that of 4-substituted phenols, but more extensive because there are two transferable hydrogen atoms and removal of both leads to stable quinones. This means that instead of the four species of the standard `square scheme’ that are formed upon PT, ET, or CPET from HX (Scheme 4), there are nine species derived from H2Q, as shown in Figure 2. This is also the case for flavins, which are discussed below. In practice, the cationic forms, H2Q?, H2Q2+ and HQ+, are not involved in typical PCET reactivity because they are high energy species under normal conditions. In the reactions of the first O bond, hydroquinones follow the patterns outlined above for phenols. In general, the pKa AlvocidibMedChemExpress HMR-1275 values for H2Q and the Chloroquine (diphosphate) molecular weight oxidation potential of HQ- fit on Hammett correlations with other 4-substituted phenols, both in aqueous117 and in organic media.116 For example, the BDFE of the first O bond in hydroquinone is 2? kcal mol-1 weaker than that of p-methoxyphenol. With hydroquinones and catechols, however, loss of H?yields the semiquinone radical that has a high propensity to lose a second H?148 Semiquinones and related species were among the first free radicals to be investigated inChem Rev. Author manuscript; available in PMC 2011 December 8.Warren et al.Pagedetail: Michaelis’ 1935 review in this journal points out that many systems commonly understood as 1e- systems can actually undergo 1e- or 1H+/1e- redox chemistry, and that the redox properties of semiquinone-type radicals are dependent upon pH ?a very early recognition of the importance of PCET in biology.149 While hydroquinones have reactivity patterns that are in part similar to phenols, with preferential loss of H? quinones have a different PCET behavior, especially in water. Quinones are typically easily reduced to semiquinone radical anions in water, without the assistance of protons, and the Q? anions are not particularly basic (Table 6). Therefore quinone cofactors can readily mediate stepwise PCET reactions, with initial electron transfer followed by proton transfer. Q/Q? interconversion is well understood using semi-classical ET theory.150 Such stepwise mechanisms have been discussed,151 and an example of stepwise PT-ET of quinones in biology is discussed in Section 6 below. The aqueous 2H+/2e- potentials of many quinones have been reported, because they are easily measured and because they are important biological cofactors (ubiquinone, for instance, is so named because it is ubiquitous). Their electrochemistry is generally well behaved,153 although there is still much to be learned in this area.154 The electrochemical data directly give an average BDFE/BDE for each quinone system (Table 5). Interestingly, the average bond strength for most quinones lies between the relatively narrow range of 68 to 75.Potential [E?(ArOH?/0)] give these molecules a strong preference to react by concerted transfer of e- and H+ (HAT). Njus and Kelley used such reasoning to conclude that Vitamin E donates H?as opposed to e- in biological reactions.135 A characteristic of these and other systems that prefer to transfer H?rather than react by stepwise paths (cf., TEMPOH above) is the very large shift of the pKa upon redox change and (equivalently) the large shift of E?upon protonation: for -tocopherol, the pKa changes by 25 units and E?changes by 1.5 V. 5.2.5 Quinones, Hydroquinones and Catechols–The PCET chemistry of hydroquinones and catechols (1,4- and 1,2-dihydroxybenzenes, respectively) is somewhat similar to that of 4-substituted phenols, but more extensive because there are two transferable hydrogen atoms and removal of both leads to stable quinones. This means that instead of the four species of the standard `square scheme’ that are formed upon PT, ET, or CPET from HX (Scheme 4), there are nine species derived from H2Q, as shown in Figure 2. This is also the case for flavins, which are discussed below. In practice, the cationic forms, H2Q?, H2Q2+ and HQ+, are not involved in typical PCET reactivity because they are high energy species under normal conditions. In the reactions of the first O bond, hydroquinones follow the patterns outlined above for phenols. In general, the pKa values for H2Q and the oxidation potential of HQ- fit on Hammett correlations with other 4-substituted phenols, both in aqueous117 and in organic media.116 For example, the BDFE of the first O bond in hydroquinone is 2? kcal mol-1 weaker than that of p-methoxyphenol. With hydroquinones and catechols, however, loss of H?yields the semiquinone radical that has a high propensity to lose a second H?148 Semiquinones and related species were among the first free radicals to be investigated inChem Rev. Author manuscript; available in PMC 2011 December 8.Warren et al.Pagedetail: Michaelis’ 1935 review in this journal points out that many systems commonly understood as 1e- systems can actually undergo 1e- or 1H+/1e- redox chemistry, and that the redox properties of semiquinone-type radicals are dependent upon pH ?a very early recognition of the importance of PCET in biology.149 While hydroquinones have reactivity patterns that are in part similar to phenols, with preferential loss of H? quinones have a different PCET behavior, especially in water. Quinones are typically easily reduced to semiquinone radical anions in water, without the assistance of protons, and the Q? anions are not particularly basic (Table 6). Therefore quinone cofactors can readily mediate stepwise PCET reactions, with initial electron transfer followed by proton transfer. Q/Q? interconversion is well understood using semi-classical ET theory.150 Such stepwise mechanisms have been discussed,151 and an example of stepwise PT-ET of quinones in biology is discussed in Section 6 below. The aqueous 2H+/2e- potentials of many quinones have been reported, because they are easily measured and because they are important biological cofactors (ubiquinone, for instance, is so named because it is ubiquitous). Their electrochemistry is generally well behaved,153 although there is still much to be learned in this area.154 The electrochemical data directly give an average BDFE/BDE for each quinone system (Table 5). Interestingly, the average bond strength for most quinones lies between the relatively narrow range of 68 to 75.
